Disclosed in Japanese patent publications, such as Patent Application KOKAI Nos. 2-23410, 2-140815, 2-264316, 3-121515 and 3-134720, are memory cards, that comprise a static RAM (SRAM) formed of a volatile semiconductor memory, a replaceable main battery as a backup power source for retaining information stored in the SRAM, and a sub-battery connected in parallel with the main battery and used as a backup power source for the SRAM when the main battery is being replaced.
Further, disclosed in Japanese patent publications, such as Patent Application KOKAI Nos. 2-76019 and 3-134720, are memory cards that comprise a battery holder, designed to contain a main battery and to be capable of being loaded into and unloaded from a memory card body, and a locking lever slidable in the memory card and designed to permit or prohibit attachment to and removal of the battery holder from the memory card body by engaging or being disengaged from the battery holder.
Also disclosed in a Japanese patent publication is, Patent Application KOKAI No. 4-104388 relating to a memory card that comprises a RAM, batteries, and an element for switching between the source voltage and an externally supplied voltage, wherein a plurality of batteries for supplying the source voltage are connected in series in the memory card. This memory card is designed to reduce the difference between the supply voltage having so far been supplied externally and the backup source voltage in the memory card, thereby avoiding the destruction of data stored in the memory when the memory card is removed from an external apparatus. Further, the batteries for supplying the backup source voltage are connected in series, so that when the batteries are dead, they must be replaced with new ones with the memory card connected to the external apparatus or the like.
Thus, in the case of a portable information devices such as memory cards, a complicated mechanism has to be used in replacing batteries while maintaining the source voltage for the volatile memory (RAM).